1. Field of the Invention
The present invention relates to transmission devices, and more particularly, to a transmission device constituting a ring network and adapted to add/drop data.
2. Description of the Related Art
Transmission devices constituting a ring network (e.g., SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) transmission devices) are generally configured in compliance with an architecture called BLSR (Bidirectional Line Switched Ring) or UPSR (Unidirectional Path Switched Ring) to cope with communication failure.
FIG. 17 is a block diagram of a conventional transmission device configured according to BLSR. As shown in the figure, the transmission device has an ADM (Add/Drop Multiplexer)/BLSR unit 200. The ADM/BLSR unit 200 includes an ADM 201, an EoS (Ethernet Over SDH) 202, switches (SW) 203 and 204, and bridges (BR) 205 and 206. In the following, explanation is made on the assumption that the network is based on SDH but applies also to the case where the network is based on SONET.
The ADM 201 adds/drops and cross-connects signals path by path. The EoS 202 converts Ethernet (registered trademark) packets received from a lower-level network into ADD path frames for the SDH network. Also, the EoS 202 converts DROP path frames of the SDH network, output from the ADM 201, into Ethernet packets, which are then output to the lower-level network.
The switches 203 and 204 individually perform Work (W)/Protection (P) route switching, on a section-by-section basis, of signals to be input to the ADM 201. The bridges 205 and 206 each operate in response to a control signal to distribute the signal output from the ADM 201 to the Work and Protection routes. In FIG. 17, the solid lines (except those extending downward from the ADM 201) represent the Work routes of the ring network, and the dashed lines represent the Protection routes.
FIG. 18 shows an exemplary configuration of a BLSR network using the conventional transmission devices. As illustrated, nodes (transmission devices) 211 through 226 are connected in a ring. Each of the nodes 211 to 226 functions as the transmission device shown in FIG. 17. The node 211 is an ADD node and the node 226 is a DROP node. The nodes 211 to 226 provide SDH/SONET transmission of Ethernet packets by means of a path (Work path) along the Work route indicated by arrows 231. Redundancy switching of the nodes 211 to 226 is executed in conformity to Bellcore GR-1230.
Also, FIG. 18 shows parts of the nodes 225 and 226 in detail, as indicated by arrows 233. The node 225 has an ADM 225a and a bridge 225b respectively corresponding to the ADM 201 and the bridge 206 shown in FIG. 17. The node 226 has an ADM 226a and a switch 226b respectively corresponding to the ADM 201 and the switch 204 shown in FIG. 17.
Let it be assumed here that FS-R (Forced Switch-Ring; path switching by the operator) is executed in the span between the nodes 225 and 226. For example, the operator operates his/her terminal such that FS-R/IDLE is output from the node 226, as indicated by arrow 234.
In response to the FS-R request, the node 225 carries out bridging. Specifically, signal (data) is distributed by the bridge 225b in the manner shown in FIG. 18, so that the data is transmitted through the Work path indicated by the arrows 231 as well as the path (Protection path) along the Protection route indicated by arrows 232. Namely, identical data arrives at the node 226 via the Work and Protection paths indicated by the arrows 231 and 232, respectively.
However, the Protection path has a transmission path length longer than that of the Work path by 15 spans (span between the nodes 225 and 224+ span between the nodes 224 and 223+ . . . + span between the nodes 211 and 226). At the node 226, therefore, a delay skew is caused between the data received from the Work path and the same data received from the Protection path. For example, where the transmission distance of one span is 80 km at a maximum, the fiber delay skew is about 6 ms at a maximum, and if the transmission delay caused per node is about 0.1 ms, then the data delay skew between the Work and Protection paths is about 7.5 ms. Thus, while the bridging is performed in the node 225, the data arriving at the node 226 via the Protection path is delayed for the data delay skew with respect to the data arriving at the same node via the Work path. At this point of time, the switch 226b of the node 226 is not yet switched to the Protection path, that is, switching is not yet executed; therefore, the node 226 drops the data received via the Work path.
FIG. 19 shows the network after the switching. In FIG. 19, like reference numerals are used to denote like elements appearing in FIG. 18, and description of such elements is omitted.
After the bridging is executed, the node 225 transmits information indicating completion of bridging (FS-R/Br) to the node 226 along the route indicated by arrow 235. On receiving the information, the node 226 causes the switch 226b to switch the DROP path from the Work path to the Protection path, so that the ADM 226a drops the data received via the Protection path, instead of the Work path.
At this time, duplicate data corresponding to the delay skew between the Work and Protection paths is dropped from the node 226. Where the transmission rate of the transmission paths is 10 Gbps, for example, 75-Mbit duplicate data is output. Such a phenomenon occurs when a BLSR is switched in response to FS-R, SD-R (Signal Degrade-Ring) or MS-R (Manual Switching-Ring). A similar problem also arises in a UPSR due to the delay caused at the time of active/standby switching.
In actual operation, duplication of data often takes place when FS-R/MS-R control is executed for the purpose of maintenance of communication lines. Data duplication in the SDH network may cause no particular problem during the maintenance. Recently, however, a major part of data conveyed over transmission paths is packet data such as Ethernet packets, and thus, data duplication causes increase in the IP traffic of networks to which the data is dropped.
Usually, duplicate packets are discarded by IP routers or the like. Depending on connected devices, however, duplicate packets may possibly cause critical failure such as data destruction or traffic disruption. Especially, where duplicate packets are generated in a transmission device constituting the backbone of a network, the network may possibly encounter a fatal failure.
For SONET/SDH rings, a means for realizing a non-momentary-interruption path by UPSR has been proposed as a means to prevent data duplication. In this connection, an optically bidirectional ring switching method has been proposed in which UPSR is fused into BLSR to realize a non-momentary-interruption path by the BLSR system, thereby providing two advantages of high line capacity efficiency and high line reliability and also making it possible to decrease memory capacity (see, e.g., International Publication No. WO 01/061937).
However, conventional transmission devices do not have a means to prevent packet duplication caused at the time of active/standby switching. Thus, to eliminate the drawback, routers or the like are connected to the ADD/DROP ports of a SONET/SDH ring, or duplicate packet filters using EoE (Ethernet over Ethernet) techniques etc. are provided at the ADD/DROP ports. It is therefore necessary that the individual ADD/DROP ports of the SONET/SDH ring should be provided with routers or filters, which lowers traffic rate and increases costs.
On the other hand, the switching method realizing a non-momentary-interruption path requires that identical data should always be passed via the Work and Protection paths, as in UPSR, and thus cannot be applied to BLSR.